Pneumatic rubber tires conventionally have treads of lug and groove configuration composed of individual tread lugs circumferentially spaced around the tire.
However, for some pneumatic tires, particularly tires for buses, it is desired that their rubber treads are configured with a lugs which comprise a plurality of circumferential continuous ribs with intervening grooves.
It is envisioned that such pneumatic tire tread configuration present challenges in a sense of providing such rib configured tread with rubber composition to promote a combination of stiffness to promote handling for the tire, reduced hysteresis (indicated by a rebound physical property) to promote reduced internal heat buildup within the tread ribs during service and resistance to abrasion (e.g. to promote resistance to tread wear at the road-contacting surface of the tread).
While it may be thought of to provide such tread rubber as a cis 1,4-polybutadiene rubber rich rubber composition to promote resistance to tread wear (resistance to abrasion) and, further, to provide a significant level of rubber reinforcing carbon black content to promote increased stiffness, it is recognized that such inclusions for a circumferential ribbed tire tread would also promote increased hysteresis for the rubber composition, and generally promotes an unwanted, internal heat buildup within the tread during tire service. Also, increase rubber processing challenges are promoted which includes processing the uncured rubber composition involving mixing the rubber composition in an internal rubber mixer and, also shaping it by extrusion processes because of expected increased viscosity of the uncured rubber composition having been promoted by both the increased carbon black and cis 1,4-polybutadiene rubber contents of the rubber composition.
For this invention, a combination of alternatives is to be evaluated for promoting both cured tread rubber properties and acceptable uncured rubber processing for preparation of a ribbed tread for heavy duty use for a bus or truck.
In one embodiment, use of a cis 1,4-polybutadiene rubber with a high heterogeneity index is proposed to promote processability of the cis 1,4-polybutadiene rubber.
In one embodiment, a controlled level of rubber reinforcing carbon black is proposed where a high reinforcing rubber reinforcing carbon black is proposed.
In one embodiment, a low level of precipitated silica is proposed to promote acceptable hysteresis in terms of rubber rebound physical property to promote lesser internal heat buildup in the tread together with an associated low level (relatively low content in terms of phr) of silica coupler to promote less viscosity buildup in the uncured rubber composition as it is processed.
In one embodiment a cure package for the rubber composition is proposed with a relatively low ratio of sulfur to sulfur cure accelerator to promote a relatively low sulfur cure density of the rubber composition.
In one embodiment, a sulfur cure accelerator combination of two primary sulfur cure accelerators comprised of two different sulfenamide based accelerators without, and therefore in an absence of, a secondary sulfur cure accelerator such as, for example, diphenyl guanidine, thiuram mono and disulfides, dithiocarbonates and ditolyguanidine, to promote a rate of cure of the rubber composition without necessitating use of a sulfur cure rate retarder to prevent scorching (preventing too high rate of vulcanization) of the rubber composition.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing of at least one elastomer blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art.